Sound fields have been used to levitate objects by taking advantage of the Boltzmann-Ehrenfest principle of adiabatic invariance, which relates the acoustic potential acting on an object positioned in a single-mode cavity to a shift in resonant frequency caused by the presence of the object. In Putterman et al., “Acoustic Levitation and the Boltzmann-Ehrenfest Principle,” J. Acoust. Soc. Am. 85(1), (1989), the torque imposed on the object in a single mode chamber by acoustic energy equals the angular derivative of the experimentally determined frequency shift (Δω). However, the levitation technique disclosed in this article is limited to high frequency sound waves used in single-mode cavities. Further, the created acoustic torque will only align with the cylindrical object being levitated in a horizontal plane, or when θ=90°. These constraints, i.e., high frequency acoustic energy, the single mode cavity and the middle horizontal plane, severely limit the applications of the disclosed levitation technique.
In an earlier work, Allen et al., “A Powerful High Frequency Siren,” J. Acoust Soc. Am. 19(5), (1947), a high frequency siren is used to generate chamber pressures of about two atmospheres and an output energy of approximately two kilowatts which can be used for heating purposes. However, this high frequency siren finds little use in most environments where such a sound output would be unacceptable or dangerous.
To date, acoustic or sound energy has not been used effectively to impart thrust or force on an object for purposes of providing useful work. While limited examples of levitation are known, the requirement that the object being levitated be disposed in an enclosed chamber renders the technique largely inapplicable. Further, the use of powerful sirens for generating heat is also largely industrially inapplicable and potentially harmful to workers.